With the growing use of portable electric products such as video cameras, mobile phones, and portable computers, significance of secondary batteries being mainly used as a power source on which the portable electric products work are increasing. In particular, a lithium secondary battery has a high energy density per unit weight and allows quick charging, when compared with other conventional secondary batteries such as a lead storage battery, a nickel-cadmium battery, a nickel-hydrogen battery and a nickel-zinc battery, and so on, and thus, its use is on an upward trend.
A lithium secondary battery has a relatively higher operating voltage compared with other secondary batteries such that it is used as a power supply of a portable electronic device with an operating voltage of 3.6V or more, or a plurality of lithium secondary batteries are connected in series with each other to thereby be used for a high output hybrid automobile.
Can-type lithium batteries using a metal can as a container mostly use a cylindrical or prismatic metal can as a container and are sealed therein by welding. The can-type secondary battery has a fixed shape and thus limits the design of an electric product which uses the battery as a power source. In addition, it is difficult to decrease the volume of the product. Therefore, a pouch-type secondary battery prepared by putting a cathode, anode, separator, and electrolyte into a pouch package formed of a film and then sealing the same has been developed and used.
A typical secondary battery pouch is formed in a multilayer structure by sequentially stacking an inner resin layer such as polyolefin-based resin layer which has thermal adhesive property to serve as sealing material, a metal foil layer such as an aluminum layer serving as both a substrate which maintains mechanical strength and a barrier layer against moisture and oxygen, and an outer resin layer such as a nylon layer serving as a substrate layer and a protective layer. The polyolefin-based resin layer may typically be formed of casted polypropylene (CPP).
The case formed of pouch is advantageous in that the shape is variable, and secondary batteries with the pouch can have the same capacity with less volume and weight, compared to secondary batteries having other case. However, since a soft material is used in a pouch, unlike the can-type case, there is a shortcoming that the mechanical strength and sealing reliability of pouch may be low.
After the pouch receives an electrode assembly and electrolyte, the pouch is heat-sealed at its periphery. In particular, recently, study for pursuing miniaturization and high-energy of secondary batteries by folding the pouch sealing portion that is not directly related with a battery capacity or an accommodating function is ongoing.
However, because the inner resin layer of the pouch melts and then crystallizes during heat-sealing of its periphery, cracking may occur by the subsequent process of folding a folding portion of the pouch. At this time, when the inner resin layer is even slightly ruptured, the metal layer of the pouch is exposed to inside of the battery, then side reaction occurs from reaction with the electrolyte. When the side reaction occurs, an active layer of the electrode is mechanically collapsed, thus facilitating water penetration such that the life of the battery is then rapidly decreased.
Accordingly, in order to prevent the breakdown of the insulation resistance due to cracks of the folding portion, an alternative such as adjusting folding angles has been suggested, but the situation is still in need of improvement.